Twenty-six new black hole candidates have been discovered in the neighbouring Andromeda galaxy. According to the astronomers involved, these could be just the tip of the iceberg. Details of the find will be published in the 20 June issue of The Astrophysical Journal.

The discoveries are the culmination of 13 years of observation. Researchers used Nasa's Chandra and the European Space Agency's XMM-Newton satellites. Both record the X-ray light emitted by celestial objects.

Black holes are the most mysterious objects in the universe. They are regions of space in which the density of matter has become so great that the gravitational field is overwhelming. They will devour anything that strays too close. Once inside, nothing can escape back into space, not even light.

So, strictly speaking you can't see black holes. You have to infer their presence from X-rays given out as they rip nearby stars to pieces. Hence the reason astronomers refer to these as black hole candidates. Even so, it's big news.

Black holes were first discovered not in space but on the pages of a notepad in the German trenches of the first world war. Physicist Karl Schwarzschild was serving as an artillery officer on the Russian front in 1915. In his spare time, he was investigating Albert Einstein's controversial new theory of gravity: general relativity.

He found an equation that suggested regions of space and time could become like lobster pots, trapping anything that fell into them. The point of no return around these "black holes" was dubbed the event horizon.

Schwarzschild completed the work shortly before facing his own event horizon. He contracted a painful skin disease called pemphigus and was returned to Germany, where he died in May 1916.

It took astronomers decades to convince themselves that black holes existed in nature. Convincing proof came in the 1970s when Nasa launched the Uhuru X-ray satellite and discovered Cygnus X-1, the first black hole candidate.

It made such headlines at the time that prog-rockers Rush (naturally) wrote a two-part epic song spanning two albums (again naturally) about it.

Black holes are no mere curiosities; they are the keys to a deeper understanding of the universe. No theory of physics can yet say what is happening inside a black hole.

General relativity suggests that Schwarzschild's event horizon is an invisible boundary that can be crossed without consequence but never returned from. Recently, however, quantum mechanics make it look as though the event horizon could be a real boundary and that crossing it will result in destruction. This has been dubbed the firewall.

Both cannot be right. One must be wrong. Whichever one is right points us towards a greater understanding of gravity and how it behaves on small scales.

While these new black holes may not immediately help in the quest for a new theory of gravity, they are nevertheless extraordinary. The more black holes astronomers identify, the more they will be able to study them and learn their secrets.